Phosphatidylinositol 3-kinase (PI 3-kinase) was originally identified as an activity associated with viral oncoproteins and growth factor receptor tyrosine kinases which phosphorylates phosphatidylinositol (PI) and phosphorylated derivatives of PI at the 3'-hydroxyl of the inositol ring [Panayotou et al., Trends in Cell Biol., 2:358-360 (1992)]. The initial purification and molecular cloning of PI 3-kinase revealed that it was a heterodimer consisting of p85 and p110 subunits [Otsu et al., Cell, 65:91-104 (1992); Hiles et al., Cell, 70:419-429 (1992)].
The p85 subunit acts to localize PI 3-kinase to the plasma membrane by the interaction of its SH2 domain with phosphorylated tyrosine residues (present in an appropriate sequence context) in target proteins [Rameh et al. Cell, 83:821-830 (1995)]. Two isoforms of p85 have been identified, p85.alpha. which is ubiquitously expressed, and p85.beta., which is primarily found in brain and lymphoid tissues [Volinia et al., Oncogene, 7:789-793 (1992)].
The p110 subunit contains the catalytic domain of PI 3-kinase and three isofoniis (.alpha., .beta. and .gamma.) of p110 have thus far been identified. p110.alpha. and .beta. associate with p85 whereas p110.gamma. which is activated by G protein .beta..gamma. subunits, does not [Stoyanov et al., Science, 269:690-693 (1995)]. The cloning of p110.gamma. revealed additional complexity within this family of enzymes. p110.gamma. is closely related to p110.alpha. and .beta. (45-48% identity in the catalytic domain), but does not make use of p85 as a targeting subunit, instead p110.gamma. contains an additional domain termed a pleckstrin homology domain near its amino terminus. This domain allows interaction with the .beta..gamma. subunits of heterotrimeric G proteins and it appears that it is this interaction that regulates its activity [Stoyanov et al., 1995]. Thus PI 3-kinases are defined by their amino acid identity or their activity. Additional members of this growing gene family include more distantly related lipid and protein kinases including Vps34, TOR1 and TOR2 of Saccharomyces cerevisiae (and their mammalian homologous such as FRAP and mTOR), the ataxia telangiectasia gene product, and the catalytic subunit of DNA dependent protein kinase. [See, generally, the review of Hunter, Cell, 83:1-4 (1995).]
The levels of phospliatidylinositol (3, 4, 5) triphosphate (PIP.sub.3), the primary product of PI 3-kinase activation, increase upon treatment of cells with a wide variety of agonists. PI 3-kinase activation is therefore believed to be involved in a range of cellular responses including cell growth, differentiation and apoptosis [Parker et al., Current Biology, 5:577-579 (1995); Yao et al., Science, 267:2003-2005 (1995)]. The downstream targets of the phosphorylated lipids generated following PI 3-kinase activation have not been well characterized. In vitro, some isoforms of protein kinase C (PKC) are directly activated by PIP.sub.3 and the PKC related protein kinase PKB has been shown to be activated by PI 3-kinase through an as-yet-undetermined mechanism [Burgering and Coffer, Nature, 376:599-602 (1995)].
PI 3-kinase also appears to be involved in a number of aspects of leukocyte activation. A p85 associated PI 3-kinase activity has been shown to physically associate with the cytoplasmic domain of CD28, an important co-stimulatory molecule for the activation of T cells in response to antigen [Pages et al., Nature, 369:327-329 (1994); Rudd, Immunity, 4:527-534 (1996)]. Activation of T cells through CD28 lowers the threshold for activation by antigen and increases the magnitude and duration of the proliferative response. These effects are linked to increases in the transcription of a number of genes including the T cell growth factor interleukin 2 (IL-2) [Fraser et al., Science, 251:313-316 (1992)]. Mutation of CD28 such that it can no longer interact with PI 3-kinase leads to a failure to initiate IL-2 production, suggesting a critical role for PI 3-kinase in T cell activation [Pages et al. 1994]. Based on studies using the PI 3-kinase inhibitor, wortmannin, there is evidence that PI 3-kinase(s) are also required for some aspects of leukocyte signalling through G protein-coupled receptors [Thelen et al., Proc. Natl. Acad. Sci. USA., 91:4960-4964 (1994)].
There thus continues to exist a need in the art for further insights into the nature, function and distribution of PI 3-kinase providing means for effecting beneficial modulation of PI 3-kinase effects.